In line with recent advancements in optical technology, various display technologies replacing a conventional cathode ray tube (CRT), such as a plasma display (PDP), a liquid crystal display (LCD), and an organic electroluminescent display (OELD), have been developed and are commercially available. Meanwhile, various polymer films such as a polarizing film, a polarizer protective film, a retardation film, a light guide plate, and a plastic substrate have been used for such display devices and there is a trend for the use of such polymer materials in a display device of which required characteristics have become highly advanced.
Currently, the most widely used polymer film for a display is a triacetyl cellulose (TAC) film which is used for a polarizing plate protective film or the like. However, the TAC film may have a limitation in that the polarizability thereof may decrease, a polarizer and the film may be separated or optical properties thereof may deteriorate when the TAC film is used over a prolonged period of time in a high-temperature or high-humidity environment. In order to resolve the foregoing limitation, a polystyrene-based polymer film, an acryl-based polymer film, such as methyl methacrylate, or a polycarbonate-based polymer film are suggested as alternatives to the TAC film. The foregoing polymer films may have excellent heat resistance. However, with respect to the polystyrene or polycarbonate film, birefringence may be generated during film alignment, thereby adversely affecting optical properties, because the polystyrene or polycarbonate film has an aromatic ring in the polymer, and with respect to the methyl methacrylate, a retardation value thereof is relatively small in comparison to the polystyrene or polycarbonate but the methyl methacrylate is insufficient to be used for a material for an optical device such as a liquid crystal device requiring high precision.
In order to address such limitations, a method of copolymerizing or blending a monomer or a polymer having positive birefringence with a monomer or a polymer having negative birefringence was suggested for a material for a polymer film having a low retardation value, as well as excellent heat resistance. A typical material according to the foregoing method may be a copolymer of benzyl methacrylate and methyl methacrylate. The benzyl methacrylate and methyl methacrylate have excellent optical properties because their retardation values approach zero. However, there is a limitation in that a curling phenomenon may be generated, in which a polarizing plate is severely bent or distorted when a polarizing film and a TAC film are laminated together, because the thermal expansion coefficients of the benzyl methacrylate and methyl methacrylate are both higher than that of the TAC film used for a polarizing plate protective film. When the foregoing curling phenomenon occurs in a polarizing plate, display quality deteriorates due to the occurrence of a light-leakage phenomenon in the polarizing plate and liquid crystals in a display panel may also be damaged. Therefore, urgent improvements to rectify the foregoing limitations are required.